The present invention relates to a spherical bearing for use in, for instance, transmission and steering wheel portion of an automobile, and a link motion mechanism of various automation machines, and more particularly to an oilless spherical bearing and a method of production thereof.
Hitherto, as a spherical bearing of this type, for instance, one shown in FIG. 11 is known (refer to the Japanese Patent Publication No. 42569/1976). Referring to FIG. 11, an inner ring 100 has an outer peripheral surface formed spherically, and the inner ring 100 is slidably fitted with an outer ring 101 whose inner peripheral surface is similarly formed spherically. In addition, a liner 102 formed of fluoride resin or the like is interposed between the inner ring 100 and the outer ring 101 to realize an oilless spherical bearing.
As illustrated in FIG. 12, such a spherical bearing is arranged as follows. The surface of the inner ring 100 is coated with a self-lubricating thin plate 103 formed of a low-friction high-polymer material, and the thus prepared inner ring 100 is accommodated in a mold 104 like a core, the mold 104 having a configuration that matches with that of the outer ring 101. A low-melting-point alloy is cast into a cavity therebetween, which completes the formation of the outer ring 101 and, at the same time, the assembly. In addition, sliding surfaces are formed between an outer surface of the inner ring 100 and the above-described self-lubricating thin plate 103 secured to the outer ring 101 at the time of casting.
Furthermore, since the free rotation of the inner ring 100 is hampered as a result of the shrinkage of the outer ring 101 during cooling and hardening, the outer ring 101 is compressed in the axial direction after casting, as shown in FIG. 13, so that the outer ring 101 is subjected to slight plastic deformation in the form of a chevron in which a central part thereof is bent in terms of its vertical cross section, and a slight gap required is hence formed between the thin plate 103 and the inner ring 100.
In such a prior art, however, bonding between the self-lubricating thin plate 103 and the outer ring 101 is effected by fusing the surface of the thin plate 103, which is brought into contact with the molten metal during casting, to the inner surface of the outer ring 101 by means of heat thereof. However, the affinity between resin and metal is generally poor, the bonding strength is weak if they are simply fused to each other, and there is a possibility that the thin plate 103 may become exfoliated due to a shearing force acting on the bonded surfaces, causing the position of the thin plate 103 to be offset. When the position of the thin plate 103 is offset, the metal surface of the outer ring 101 is exposed, and the metallic portion is brought into direct contact with the inner ring 100, which results in increased sliding resistance and causes rattling due to the partial wear of the sliding surface.
In addition, since the liner 102 is formed of a resin, the compressive strength thereof is low, and has the possibility of becoming damaged if the compressive load applied to the inner ring 100 becomes large. Moreover, even if damage does not result, there is a problem in that crip deformation may occur, making it impossible to bear a large load.
Furthermore, since the fluoride-based resin has a small conductivity, the fluoride-based resin is unable to allow the heat generated by friction with the inner ring 100 during use to escape effectively, so that there has also been the problem of the sliding surface becoming overheated and seized.
Meanwhile, in production, the thin plate 103 is coated on the surface of the inner ring 103, but, during casting of the outer ring 101, it is necessary to prevent the molten metal from flowing to the side of the inner ring 100. Namely, if the molten metal enters between the thin plate 103 and the inner ring 100, the molten metal hardens, with the result that a metallic foreign substance is interposed between the thin plate 103 and the inner ring 100 and the surface of the inner ring 100 becomes worn by the metallic foreign substance during use. In addition, the thin plate 103 also becomes damaged by wearing powders, so that the characteristics of the bearing are lost.
This problem would be satisfied with a technique that the thin plate 103 is held in close contact with the outer surface of the inner ring 100 during casting, but since the outer surface of the inner ring 100 is spherically shaped, it has been difficult in terms of molding to hold the sheet-like thin plate 103 in close contact with such a spherical portion. For instance, even if the thin plate 103 is formed into a spherical shape in advance, if an attempt is made to insert the inner ring 100 into such a spherically formed one, the inner ring 100 cannot be inserted since its central portion is expanded. If an attempt is made to insert it forcedly, there has been the problem that the thin plate 103 becomes broken. In addition, if the thin plate 103 is formed into the shape of a strip and if the thin plate 103 is wound around the outer surface of the inner ring 100, and the both ends of the wound thin plate 103 are provisionally attached to each other by means of an adhesive tape, there has been the problem that the molten metal flows round to the side of the inner ring 103 from the seam of the thin plate 103.
On the other hand, in a conventional example, since a very small gap is formed between the thin plate 103 and the inner ring 100 by applying an axial external force to the shrunk outer ring 101, it has been impossible to form this small gap uniformly. In other words, the outer ring 101 has been bent into the chevron shape by an external force, the very small gap is large at the central portion of the inner peripheral surface of the outer ring 101 and is small at edge portions 101a,101a. For that reason, the swinging resistance of the inner ring 100 becomes nonuniform, and the smooth movement of the inner ring 100 is hampered. In addition, since the gap is nonuniform, the edge portions of the thin plate 103 are partially brought into contact with the outer surface of the inner ring 100 when the load is applied. As a result, the contact area between the thin plate 103 and the inner ring 100 is narrowed, and a concentrated load is applied to the vicinity of the edge portions, making it impossible to bear a high load. In addition, there have also been such problems that partial wear is likely to occur at the edge portions, resulting in play. Furthermore, it has been unavoidable to increase the very small gap in order to prevent the partial contact between the thin plate 103 at the edge portions 101a,101a and the inner ring 100, so that there has been another problem that this results in a weakness against an impact load as well as a large amount of play, possibly causing a delay in the transmission of a force in, for instance, a link mechanism.